> Just out of curiosity (this being a subject I know
>little about), the chances of a direct collision
>notwithstanding, how close would another star have
>to pass to our solar system to be able to exert any
>force on us at all? Is there any chance of such a
>thing dramatically altering - say - the orbits of
>the planets in our solar system?

Well, just as a start, assuming an average sized star (of which our Sun is a good example) it'd have to pass at least as close as the nearest star to our Sun now, which is about 4 light years away. And in fact, the Alpha Centauri system is really a triple star system. Two of the stars, Alpha Centauri A and B, are a typical binary pair and are each about the size of our Sun if memory serves (and I'm doing all of this from memory, so if you look it up and prove me wrong, I'll just say "oooh, look who knows so much!") The third star, Proxima Centauri, is a white dwarf. So right there we've got three stars sitting at about 4 light years distant that have negligible effects on the orbit of the planets of this system. Discounting the effects of Proxima, we can say that there are two stars about the size of our own Sun (or, abstracting, one star twice the size of the Sun) about 4 light years away.

One star twice the size of our Sun at 4 light years is equivalent to one star the size of our Sun at 2 light years. So one star about 2 light years away would have the same effect that Alpha Centauri has on us now--in other words, a negligible effect.

So, as a starter, we can say that an average sized star would have to come closer than 2 light years from us to have any real effect. To figure out exactly how close would require more math than I'm willing to do (I had to do enough to convince myself that the previous paragraph was true, what with gravity following an inverse square law, and me being generally untrusting of my own instincts when math is concerned) so I'm going to quit now.

-- Dave